421-a: Methodology

We will soon be releasing all the data sets used for our 421-a visualizations. Stay tuned!

Contribute to our analysis

These visualizations are based on several relevant and available public data sets (see data sources below). But these data sets do not capture all the information we would like to incorporate into our analysis. If you have access to data sets and other information that would enhance our work, we would like to hear from you. Our mission is to provide the most accurate and up-to-date information possible. We are particularly interested in the following:

  • More accurate “start” date. Our maps do not account for undue delay provisions, construction extenders, grandfather clauses, etc. These transition mechanisms were not included primarily because a systematic data set that captures such information could not be identified.
  • Affordable housing statistics. The affordable housing for any property is based on the 421-a regulations in effect upon the same year as the “Year Built” for that property. For off-site affordable housing, we assumed the equivalent of 20% of all units were created off-site. That being said, the exact number of new affordable housing units may be more depending on the affordability of such off-site units, but a public data set that best captures this information was not identified. Moreover, a public data set showing the location of the off-site units was not identified.
  • The 25-year exemption. Our maps do not account for the certain exceptions to 421-a, primarily because a data set that captures this information could not be identified. Specifically, before 2008, property located outside the GEA was eligible for a 25-year exemption without any affordable housing requirement if it was either: located in a “Neighborhood Preservation Program” area; located in an area eligible for “Rehabilitation Mortgage Insurance Corporation” loans; or part of a government-assisted project. We could not identify a data set that captures this information. For the NPP, however, in absence of any explicit boundaries for 421-a, we used the boundaries set forth by the Department of Housing Preservation and Development in Title 28, Section 5-10 of the Rules of the City of New York.


Generally, we applied the 421a regulations that corresponded to the “Year Built” field, as recorded in MapPLUTO by the Department of City Planning. In years in which there were significant changes to 421a regulations, we applied the new regulations to the following year.

See detailed analysis notes